The present invention relates to packet communication methods, networks, and computer program products, and, more particularly, to communication methods, networks, and computer program products for communicating time division multiplexing (TDM) traffic and data (STATMUX) traffic over a single connection, where the single connection may comprise multiple aggregated lines.
There have been a number of recent enhancements that have improved the ability to deliver services over access networks. Many of these techniques have dealt with mechanisms for delivering data services, although in certain cases advances have addressed TDM services instead. An example of such techniques has been introduced in IEEE 802.3 for delivering native Ethernet over one or more DSL lines, also known as the Ethernet in the First Mile (EFM) project, or IEEE 802.3ah.
In those cases where both data and TDM services have been addressed simultaneously, they have been primarily addressed via an overlay network—running TDM over a data network, or data over a TDM network. Although this can be successful and desirable in some cases, it does create a new service type that generally differs from its original service via some important characteristics. For example, when delivering TDM services over a data network, the TDM service often has significantly different jitter and synchronization characteristics than a traditional TDM service delivered over a synchronous network. Examples of conventional approaches for delivering data services, TDM services, and both data and TDM services over one or more access lines will now be described.
Data Service Delivery
Early ITU standards for delivering data services over access lines used one of two alternatives: HDLC or ATM. Both of these techniques have generally fallen out of favor for the more efficient techniques discussed below.
Standards from both the IEEE and ITU have generally improved the ability to deliver data transport over access systems. The primary advances initiated by these standards are:                a) An encapsulation technique known as 64/65-octet encapsulation was introduced in IEEE 802.3, which allowed a generally efficient and predictable method for delivering variable size Ethernet packets over an xDSL system. Whereas previous data encapsulation techniques typically had variable and potentially high overhead, this technique is very efficient for data transport.        b) A bonding technique known as Ethernet bonding was introduced in IEEE 802.3, which allowed multiple access lines of various and distinct qualities to be aggregated together into one virtual Ethernet interface. This technique was the first to allow variable rate access lines to be aggregated together, and to provide a dynamic and flexible environment in which the impact of a failure or addition of a line was relatively small. This technique was generalized in ITU G.998.2 to apply to more generic access technologies than the ones covered in IEEE 802.3.        c) The 64/65-octet encapsulation techniques were expanded in ADSL2 Amendment 1 to apply to several access technologies, and to allow, among other things, pre-emption. Pre-emption ensures that high priority data does not have to wait behind lower priority data, thus allowing that data to jump to the head of the line and have a much lower latency. Latency is typically a key metric for TDM delivery.All of these techniques have combined to offer a generally high-speed, efficient, flexible, and reliable access network for data services.        
At the same time these standards were developing for delivery of data natively over xDSL access lines, the ITU was developing standards for delivering data over TDM (SONET/SDH) infrastructures. Two methods developed for encapsulating data over SONET/SDH in a flexible manner are X86 and the Generic Framing Protocol (GFP).
TDM Service Delivery
TDM services have historically been delivered over xDSL technologies such as HDSL [HDSL], and SHDSL [SHDSL]. When delivering TDM services, the access line runs at the exact rate required for that TDM service. For example, a 2048 Kbps E1 service is delivered over an access line with a 2048 Kbps payload rate. The basic traditional techniques allow for exactly one TDM service per line.
To improve the reach of the TDM service, methods were introduced to run the TDM service over two lines (called 4-wire mode in SHDSL). In this case, each line delivered half the payload rate of the desired end service (e.g. each line ran at 1024 Kbps for a 2048 Kbps service).
TDM services have also been enhanced with a multi-line aggregation strategy with Virtual Concatenation (VCAT) and Link Capacity Adjustment Scheme (LCAS). These techniques allow for combining, or concatenating, multiple lines into a single aggregate connection. The primary application of VCAT/LCAS is as an underlying aggregation technique for GFP, where a single, large multi-line pipe is created for a data service. For example, five (5) E1s, each at 2 Mbps, can be combined into a single 10 Mbps data service.
Data and TDM Service Delivery
Access networks deliver a large amount of voice/TDM services in addition to an ever-increasing amount of data services. In some cases, these TDM applications can be delivered over the packet network using techniques, such as Voice Over IP (VoIP) and Pseudowire End-to-End Emulation (PWE3). These techniques use a packet network to deliver voice services, and can be a sufficient delivery mechanism for some TDM applications.
There are some cases, however, where emulation of TDM services over a packet network may result in insufficient performance. This is true with both VoIP and PWE3. One such application is in the area of backhaul for cellular services. Cellular equipment has been designed and built based upon the performance characteristics of traditional TDM access services, such as a T1 line. The performance characteristics of an emulated T1 service can be significantly different than those for the traditional T1 service, and replacing a traditional service with an emulated service can result in serious problems for the mobile backhaul network.
Additionally, for transport operators that “sell” a service to another provider, an emulated TDM service cannot be sold as a native TDM service because of those performance differences. An emulated TDM service would most often have to be priced and sold differently than a native TDM service. For these reasons and others, simple emulation of a native TDM service is often unsatisfactory.
A rarely used historical technique for the simultaneous delivery of TDM and data services over xDSL is known as dual-bearer mode. Dual-bearer mode partitions a single physical DSL line into two parts: one part transporting TDM, and one part transporting data. Dual-bearer mode may suffer from several limitations that may make it difficult to use in a multi-line environment:                a) Dual-bearer mode is defined to operate on a single line. Although it can certainly be independently applied to each line in a multi-line group, there are no defined algorithms for partitioning one or more TDM circuits over multiple dual-bearer lines.        b) Dual-bearer mode requires synchronization. The TDM slots on each line (in 2-pair/4-wire mode) are reserved, and any multi-line distribution algorithm would involve synchronization between the distribution algorithm and the lines over which it is transmitting.        c) Dual-bearer mode statically reserves bandwidth for TDM on each line. In the event of line failures, there is no process to move or re-distribute the TDM traffic over the multiple lines in a simple fashion.        d) Because of the static partitioning of TDM and data applications, the addition or removal of TDM bandwidth from a dual-bearer line may be difficult and must be synchronized and coordinated at both ends of each line, introducing a complicated coordination process.        